Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another; and joining with it the fact that among hermaphrodite organisms, the germ-cells developed in any individual, are usually not fertilized by sperm-cells developed in the same individual; we see reason for thinking that the essential thing in fertilization, is the union of specially-fitted portions of different organisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same individual, or those of different individuals. But the circumstance that there exist in such organisms, elaborate appliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the desideratum. Now this is just what the foregoing hypothesis implies. If, as was concluded, fertilization has for its object the disturbance of that approximate equilibrium existing among the physiological units separated from an adult organism; and if, as we saw reason to think, this object is eflPected by mixture with the slightly- different physiological units of another organism; then, we at the same time see reason to think, that this object will not be effected by mixture with physiological units belonging to the same organism. Thus, the hypothesis leads us to expect such provisions as we find exist.
§ 94. But here a difficulty presents itself. These propositions seem to involve the conclusion, that self-fertilization is impossible. It apparently follows from them, that a group of physiological units from one part of an organism, ought to have no power of altering the state of approaching balance in a group from another part of it. Yet self-fertilization does occur. Though the ovules of one plant, are generally fertilized by pollen from another plant of the same kind; yet they may be, some of them, fertilized by the pollen of the same plant. And though, among hermaphrodite animals, self-fertilization is usually negatived by structural or functional arrangements; yet in certain Entozoa, there appear to be special provisions by which the sperm-cells and germ-cells of the same individual may be united, when not previously united with those of another individual. Certainly, at first sight, these facts do not consist with the above supposition. Nevertheless, there is a satisfactory solution of them.
In the last chapter, when considering the variations that may result in offspring from the combination of unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly- different parents, there cannot be maintained an even distribution of the two orders of units. We saw that the instability of the homogeneous, negatives the uniform blending of them; and that, by the process of differentiation and integration, they must be more or less separated; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent: an inference which harmonizes with daily observation. And we also saw, that the sperm-cells or germcells produced by such an organism, must, in virtue of these same laws, be more or less unlike one another. It was shown that through segregation, some of the sperm-cells or germcells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side: a cause which accounts for the unlikenesses among offspring simultaneously produced. Now from this segregation of the different orders of physiological units, inherited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. If the physiological units contained in the sperm- GENESIS, UERKDITY, AND VARIATION. 281 cells and germ-cells of the same flower, are not quite homogeneous—if in some of tlie ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent; and if the like is true of the pollen-cells; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells, in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. Between part of the sperm-cells and part of the germ-cells, the community of nature will be such that fertilization will not result from their union; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. The facts, so far as they are known, seem in harmony with this deduction. Self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. Though some of the ovules produce seeds, yet more of them than usual are abortive. From which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization; since, being more prolific, these have, other things equal, greater chances in the " struggle for existence."
Further evidence is at hand in support of this interpretation. There is reason to believe that self-fertiKzation, which at the best is comparatively inefficient, loses all efficiency in course of time. After giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers; and after quoting Prof. Huxley to the effect that among hermaphrodite animals, there is no case in which '' the occasional influence of a distinct individual can be shown to be physically impossible; '' Mr Darwin writes — " from these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature. * * * in none, as I suspect, can self-fertilization go on for perpetuity. '^ This conclusion, based wholly on observed facts, is just tlie conclusion to which the foregoing argument points. That necessary action and the re-action between the parts of an organism and the organism as a whole — that power of the aggregate to re-mould the units, which is the correlative of the power of the units to build up into such an aggregate; implies that any differences existing between the units inherited by an organism, must gradually diminish. Being subject in common to the total forces of the organism, they will in common be modified towards congruity with these forces; and therefore towards likeness with each other. If, then, in a self- fertilizing organism and its self- fertilizing descendants, such contrasts as originally existed among the physiological units, are progressively obliterated — if, consequently, there can no longer be a segregation of different physiological miits in different sperm -cells and germ-cells; self-fertilization will become impossible: step by step the fertility wdll diminish, and the series will finally die out.
And now observe, in confirmation of this view, that selffertilization is limited to organisms in which an approximate equilibrium among the organic forces, is not long maintained. While growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected: like forces acting on the unlike units, will tend to segregate them, so long as continuance of evolution permits further segregation; and only when further segregation cannot go on, will the like forces tend to assimilate the units. Hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some generations; but it will be impossible in organisms distinguished by a sustained moving equilibrium.
§ 95. The interpretation which it affords of sundry phenomena familiar to breeders of animals, adds probability to the hypothesis. Mr Darwin has collected a large " body of facts, GENESIS, IIEHKDirV, AND VAllI A'lION. 283 sliovviiig, in iiccordiuico with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility," — a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. Have we not here a solution of these facts? Relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. Animals of different varieties must be those whose physiological units are more unlike than usual. In the one case, the unlikeness of the units ma}'" frequently be insufficient to produce fertilization; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous development. In the other case, both fertilization and vigorous development will be made probable.
Nor are we without a cause for the irregular manifestation of these general tendencies. The mixed physiological units composing any organism, being, as we have seen, more or less segregated in the reproductive centres it throws off; there may arise various results, according to the degrees of difference among the units, and the degrees in which the units are segregated. Of two cousins who have married, the common grandparents may have had either similar or dissimilar constitutions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. Or the brothers and sisters from whom these cousins descended, instead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees: in which last case, intermarriages among the grandchildren will be less likely to prove infertile. Or the brothers and sisters from whom these cousins descended, may severally have married persons very like, or very unlike, themselves; and from this cause there may I I have resulted, either an undue likeness, or a due unlikeness, between the married cousins. These several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. Moreover, differences of seo:re2'ation will make the reproductive centres produced by the same nearly- related organisms, vary considerably in their amounts of unlikeness; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertilization will be effective in various degrees. Hence it may happen that among offspring of nearly- related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. So that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour; and why the effect cannot be a uniform effect, but only an average effect.
§ 9(3. While, if the foregoing arguments are valid, gamogenesis has for its main end, the initiation of a new development by the overthrow of that approximate equilibrium arrived at among the molecules of the parent-organisms; a further end appears to be subserved by it. Those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform; while those organisms that have highly-complex and variable conditions of life, habitually multiply by gamogenesis. Now if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different: the aggregates of incident forces cannot be alike for all the scattered individuals. Hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied, tends to become fitted for the particular habits which its particular conditions necessitate; and in so far, unfitted for the average habits proper to the species. But these undue specializations are continually checked by gamogenesis. As Mr Darwin remarks — '* intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the variety, true and uniform in character:" the idiosyncratic divergences obliterate each other. Gamogenesis, then, is a means of turning to positive advantage, the individual differentiations which, in its absence, would result in positive disadvantage. Were it not that individuals are ever being made unlike each other by their unlike conditions, there would not arise among them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. And were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation.
This truth will be most clearly seen if we reduce it to its purely abstract form, thus: — Suppose a quite homogeneous species, placed in quite homogeneous conditions; and suppose the constitutions of all its members in complete concord with their absolutely-uniform and constant conditions; what must happen? The species, individually and collectively, is in a state of perfect moving equilibrium. All disturbing forces have been eliminated. There remains no force which can, in any way, change the state of this moving equilibrium; either in the species as a whole or in its members. But we have seen [First Principles^ § 133) that a moving equilibrium is but a transition towards complete equilibration, or death. The absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces that can cause changes in the conditions of its members — is the absence of all forces which can initiate new organisms. To say, as above, that complete molecular homogeneity existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying, that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism, no force by which it differs from any other — no force which any other does not meet with an exactly equal force — no force which can set up a new evolution among the units of any other.
And so we reach the remarkable conclusion, that the life of a species, like the life of an individual, is maintained by the unequal and ever- varying actions of incident forces on its different parts. An individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life consists of; and similarly, an absolutely- uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. Just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indirectly produced by other incident forces; and just as the combination of rhj^thmical functions thus maintained, constitutes the life of the organism; so, in a species, there is, through gamogenesis, a perpetual neutralization of those contrary deviations from the mean state, which are caused in its different parts b}^ different sets of incident forces; and it is similarly by the rhythmical production and compensation of these contrary deviations, that the species continues to live. The moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually- dissipated forces continually re-supplied from without. Besides owing to the external world, those energies which, from moment to moment, keep up the lives of its individual members; ever}^ species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations.
§ 97. What evidence still remains, may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. Let us contemplate the facts in their synthetic order.
GENESIS, IIEUEDITY, AND VARIATION. 287 That compounding and re-compounding through which wc pass from the simplest inorganic substances to the most complex organic substances, has several concomitants. Each successive stage of composition, presents us with atoms that are severally larger or more integrated, that are severally more heterogeneous, that are severally more unstable, and that are more numerous in their kinds (First Principles, § 111). And when we come to the substances of which living bodies are formed, we find ourselves among multiplied, divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. There is no reason to assume that this process ends with the formation of those complex colloids which characterize organic matter. A more probable assumption is, that out of the complex colloidal atoms, there are evolved, by a still further integration, atoms that are still more heterogeneous, and of kinds that are still more multitudinous. What must be their properties? Already the colloidal atoms are extremely unstable — capable of being variously modified in their characters by very slight incident forces; and already the complexity of their polarities prevents them from readily falling into those positions of polar equilibrium which result in crystallization. 'Now the organic atoms composed of these colloidal atoms, must be similarly characterized in far higher degrees. Far more numerous must be the minute changes that can be wrought in them by minute external forces; far more free must they remain for a long time to obey forces tending to re-distribute them; and far greater must be the number of their kinds.
Setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of w^hich the general law of Evolution thus leads us to anticipate; we get an insight into the phenomena of Genesis, Heredity, and Variation. If each organism is built of certain of these highly-plastic units peculiar to its species — units which slowly work towards an equilibrium of their complex polarities, in producing an aggregate of the specific 288 THE INDUCTIONS OF mOLOGY.
structure, and whicli are at the same time slowly modifiable by tbe re-actions of this aggregate — we see why the multiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed.
Heredity, as shown not only in the repetition of the specific structure, but in the repetition of ancestral deviations from it, becomes a matter of course; and it falls into unison with the fact that, in various simple organisms, lost parts can be replaced, and that, in still simpler organisms, a fragment can develop into a whole.
While an aggregate of physiological units continues to grow, by the assimilation of matter which it moulds into other units of like type; and while it continues to undergo changes of structure; no equilibrium can be arrived at between the whole and its parts. Under these conditions, then, an un-difierentiated portion of the aggregate — a group of physiological units not bound up into a specialized tissue — will be able to arrange itself into the structure peculiar to the species; and will so arrange itself, if freed from controlling forces, and placed in fit conditions of nutrition and temperature. Hence the continuance of agamogenesis in little- differentiated organisms, so long as assimilation continues to be greatly in excess of expenditure.
But let growth be checked and development approach its completion — let the units of the aggregate be severally exposed to an almost constant distribution of forces; and they must begin to equilibrate themselves. Arranged as they will gradually be, into comparatively stable attitudes in reLition to each other, their mobility wall diminish; and groups of them, partially or wholly detached, will no longer readily rearrange themselves into the specific form. Agamogenesis will be no longer possible; or, if possible, will be no longer eas}'.
"When we remember that the force which keeps the Earth in its orbit, is the gravitation of each particle in the Earth towards every one of the group of particles existing 91,000,000 of miles off; we cannot reasonably doubt, that each unit in GENESIS, HEREDITY, AND VAIIIATION. 289 an organism, acts, by its polar forces, on all the other units, and is re-acted on by them. When, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals, have their atoms re- arranged by forces radiated in the dark from adjacent objects; we are obliged to conclude that the excessively-unstable units of which organisms are built, must be sensitive in a transcendant degree, to all the forces pervading the organisms composed of them — must be tending ever to re-adjust, not only their relative positions, but their molecular structures, into equilibrium with these forces. Hence, if aggregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different; and they will become the more different the more widely the re- actions of the aggregates upon them differ, and the greater the num -ber of generations through which these different re-actions of the aggregates upon them are continued.
If, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another; it becomes comprehensible that when groups of units derived from two individuals are united, the group formed will be more unstable than either of the groups was before their union: the mixed units will be less able to resist those re- distributing forces which cause evolution; and may so have restored to them, the capacity for development which they had lost.
This view harmonizes with the conclusion which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells; but depends on their derivation from different individuals. It explains the fact that nearly-related individuals are less likely to have offspring than others; and that their offspring, when they have them, are frequently feeble. And it gives us a key to the converse fact, that the crossing of varieties results in unusual fertility and vigour.
Bearing- in mind that the slightly- different orders of physiological units which an organism inherits from its parents, are subject to the same set of forces; and that when the organism is fully developed, this set of forces, becoming constant, tends slowly to re-mould the two orders of units into the same form; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. In long-lived creatures that have tolerably-definite limits of growth, this assimilation of the somewhat- unlike physiological units, is liable to go on to an appreciable extent; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. And where the assimilation is not considerable, the segregation of mixed units, may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs; and several generations of selffertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminished, that they will no longer do this. The same principles explain for us the variable results of union between nearlyrelated organisms. According to the contrasts among the physiological units they inherit from parents and ancestors; according to the unlike proportions of the contrasted units which they severally inherit; and according to the degrees of segregation of such units in different sperm-cells and germ-cells; it may happen that two kindred individuals will produce the ordinary number of offspring, or will produce none; or will at one time be fertile and at another not; or will at one time have offspring of tolerable strength, and at another time feeble offspring.
To the like causes are also ascribable the phenomena of Variation. These are unobtrusive while the tolerably-uniform conditions of a species maintain tolerable uniformity among the physiological units of its members; but they become obtrusive when differences of conditions, entailing considerable functional differences, have entailed decided differences among the physiological units; and when the different physiological units, differently mingled in every individual, come to bo variously segregated and variously combined.
Did space permit, it might be shown that this hypothesis is a key to many further facts — to the fact that mixed races are comparatively plastic under new conditions; to the fact that pure races show predominant influences when crossed with mixed races; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy — have functions less-easily thrown out of balance. But without further argument, it will, I think, be admitted, that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena that seem so little allied, is strong evidence of its truth. And such evidence gains greatly in strength on observing that this hypothesis brings the facts of Genesis, Heredity, and Variation into harmony wath first principles. When we see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more unstable, and more multiform atoms, as would result from continuance of the steps through v/hich organic matter is reached — when we see that the differentiations of them assumed to occur in differently- conditioned aggregates, and the equilibrations of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force — when we see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species — and when we thus see that these apparently-exceptional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of Evolution; we have weighty reasons for entertaining the hypothesis w^hich affords us this interpretation.
CHAPTEE XI.